Molecular ion spectroscopy of BaCl$^+$*

We demonstrate a simple technique for molecular ion spectroscopy. BaCl$^+$ molecular ions are trapped in a linear Paul trap in the presence of a room-temperature He buffer gas and photodissociated by driving an electronic transition from the ground X$^1\Sigma^+$ state to the repulsive wall of the A$^1\Pi$ state. The photodissociation spectrum is recorded by monitoring the induced trap loss of BaCl$^+$ ions as a function of excitation wavelength. Accurate molecular potentials and spectroscopic constants are determined. Comparison of the theoretical photodissociation cross-sections with the measurement shows excellent agreement. This study represents the first spectroscopic data for BaCl$^+$ and an important step towards the production of ultracold ground-state molecular ions. Future steps include investigating a strong predissociation channel between the first excited $^1\Sigma$ and A$^1\Pi$ states where it is expected that the rovibrational resolution afforded by predissociation spectroscopy will allow us to efficiently measure molecular ion rovibrational temperatures.